China plans to build an environmental monitoring station on a South China Sea shoal at the heart of a territorial dispute with the Philippines, potentially raising new concerns over Beijing’s actions to assert its claims in the strategically crucial waterbody.

The top official in Sansha, which administers China’s island claims, was quoted by the official Hainan Daily newspaper as saying such stations were being built on six islands and reefs, including Scarborough Shoal off the northwestern Philippines.

Sansha Communist Party Secretary Xiao Jie said preparatory work on the stations was among the government’s priorities for 2017, but gave no other details.

Beijing seized Scarborough in 2012 after a prolonged standoff with Philippine vessels. Taiwan also includes the island in its South China Sea claims.

The other stations mentioned by Xiao would be situated on features in the Paracel island group that China has controlled since seizing parts from Vietnam in 1974.

Also this week, the commander in chief of China’s navy, Vice-Admiral Shen Jinlong, noted improving relations in a meeting with his Vietnamese counterpart, Rear Admiral Pham Hoai Nam, in Beijing.

China and Vietnam have had long-running territorial disputes in the South China Sea. Tensions spiked in 2014 after China parked an oil rig near Vietnam’s central coast, sparking mass protests in Vietnam.

The two navies and their countries should “together play a positive role in maintaining peace and stability in the South China Sea,” Shen was quoted as saying by China’s defence ministry.

South China Sea tensions have eased somewhat since Beijing erupted in fury last year after a Hague-based arbitration tribunal ruled on a case filed by the Philippines, invalidating China’s sweeping territorial claims and determining that China violated the rights of Filipinos to fish at Scarborough Shoal.

China has since allowed Filipino fishermen to return to the shoal following Philippine President Rodrigo Duterte’s calls for closer ties between the countries, but it does not recognise the tribunal’s ruling as valid and insists it has historical claims to almost the entire South China Sea, through which an estimated US$5 trillion in global trade passes each year.

China’s creation of seven man-made islands in the disputed Spratly group, complete with airstrips and military installations, has drawn criticism from the United States and others and focused attention on Beijing’s long-term plans for Scarborough.

US diplomats have said privately that reclamation work on the shoal would be seen as crossing a red line because of its proximity to the main Philippine islands.

During his Senate confirmation hearing for secretary of state, Rex Tillerson compared China’s island-building and deployment of military assets to Russia’s 2014 annexation of Crimea, and suggested China’s access to the island should not be allowed.

The topic is likely to be high on the agenda when Tillerson visits Beijing for talks with top officials on Saturday and Sunday.

China, the Philippines, Vietnam, Taiwan, Malaysia and Brunei have long contested ownership of the South China Sea, which straddles one of the world’s busiest sea lanes and is believed to sit atop vast deposits of oil and gas.